In a game so evenly matched, that could be the difference between the end of a season and a trip to the Super Bowl.

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In fact, a post-whistle penalty played a crucial part in the first meeting. The Seahawks led San Francisco 12-3 late in the third quarter, but they faced a third-and-28. Quarterback Russell Wilson completed a 15-yard pass, and Seattle was going to punt. That is, until 49ers linebacker Aldon Smith shoved his hand in Breno Giacomini’s face mask after the whistle.

The resulting 15-yard penalty gave the Seahawks an automatic first down. They capitalized by scoring a touchdown to take control of the game.

“Both these teams have a massive, massive chip, like they’re the big boy on the block,” said former scout John Middlekauff. “It’s that Pete Carroll, Jim Harbaugh way. Both of them carry themselves like they’re the No. 1 team in the country, and they know it. Who’s going to get a stupid personal-foul penalty? To me, it’s really going to come down to mental awareness and mental focus. Someone’s going to crack.”

The Seahawks faced a similar situation against St. Louis in the season finale. The Rams tried to puff their chests and play tough. Instead, they were flagged for four personal fouls in the span of two plays. The Seahawks generally kept their composure.

The 49ers went through it last week against Carolina. The Panthers helped spot San Francisco a 6-0 lead with two defensive penalties — an unnecessary roughness penalty and a head butt. The game remained chippy throughout.

The Seahawks and 49ers both like to talk after the whistle, and that intensity should be ratcheted up to another level Sunday.

“It just comes down to poise,” Seahawks tight end Zach Miller said. “You have to understand the situation and keep your emotions in check. You want to play with fire and passion, but when the whistle blows you have to shut it down — no matter what happens.”

Added Wagner, who was called for a personal foul after the whistle against New Orleans: “You’ve got to make sure you don’t do anything that costs your team the game.”